In recent years, in compression refrigerators such as air conditioners, car air conditioners and the like, a C1 or C2 saturated fluorinated hydrocarbon (HFC), which is a refrigerant that does not deplete an ozone layer has been used. However, recently, these compression refrigerators have been used under severe conditions for more and more longer time.
Accordingly, as a lubricating oil composition for a compression refrigerator like this, a lubricant oil excellent in the thermal and oxidation stability is in demand.
Further, in various compression refrigerators including air conditioners, as the HFC refrigerant that does not deplete an ozone layer, mixed refrigerants containing difluoromethane (R32) such as R410A, R407C and so on have been used.
However, in addition to protection of the ozone layer, prevention of global warming is required, and it has been desired to use a refrigerant containing much more R32 that has lower global warming potential.
The R32 has relatively high theoretical COP and thermal conductivity and low pressure loss of a refrigerant; accordingly, when used in an air conditioner, the air conditioner is characterized by high energy efficiency.
However, the R32 is characterized in that the discharge temperature of a compressor is higher by about 20° C. than that of conventional R410A or R407C.
From a situation like this, an improvement in a structure of a refrigerator as shown in Patent Document 1 for example has been under study.
Further, as a lubricant oil for a compression refrigerator that uses these refrigerants, a lubricating oil composition having high thermal and oxidation stability under the presence of these refrigerants is in demand.
The requirement like this was considered that it can be handled, as described in Patent Documents 2 to 4, for example, by blending an existing antioxidant or acid scavenger that is used in R134a or the like in a saturated HFC that is high in the stability different from an unsaturated HFC or an iodide. However, even when an existing antioxidant or acid scavenger is simply compounded or a compounding amount thereof was increased, an improving effect thereof is low.